High temperature superconductor (HTS) materials provide a means for carrying extremely large amounts of current with extremely low loss. HTS materials lose all resistance to the flow of direct electrical current and nearly all resistance to the flow of alternating current when cooled below a critical temperature. The development of HTS wires (the expression “wires” is used here for a variety of conductors, including tape-like conductors) using these materials promises a new generation of high efficiency, compact, and environmentally friendly electrical equipment, which has the potential to revolutionize electric power grids, transportation, materials processing, and other industries. However, a commercially viable product has stringent engineering requirements, which has complicated the implementation of the technology in commercial applications.
In second generation HTS wire (coated conductor) technology, currently under development, the HTS material is generally a polycrystalline rare-earth/alkaline-earth/copper oxide, e.g. yttrium-barium-copper oxide (YBCO). The current carrying capability of the HTS material is strongly related to its crystalline alignment or texture. Grain boundaries formed by the misalignment of neighboring crystalline superconductor grains are known to form an obstacle to superconducting current flow, but this obstacle decreases with the increasing degree of alignment or texture. Therefore to make the material into a commercially viable product, e.g. an HTS wire, the superconducting material must maintain a high degree of crystalline alignment or texture over relatively long distances. Otherwise, the superconducting current carrying capacity (critical current density) will be limited.
A schematic of a typical second-generation HTS wire 100 is shown in FIG. 1. The wire includes substrate 110, buffer layer 120 (which could include multiple buffer layers), superconductor layer 130, and gap or cap layer 140, and is fabricated as described below. It should be noted that in this and all subsequent figures, the dimensions are not to scale. Superconductor materials can be fabricated with a high degree of crystallographic alignment or texture over large areas by growing a thin layer 130 of the material epitaxially on top of a flexible tape-shaped substrate 110 and buffer layer 120, which are fabricated so that the surface of the topmost layer has a high degree of crystallographic texture at its surface. When the crystalline superconductor material is grown epitaxially on this surface, its crystal alignment grows to match the texture of the substrate. In other words, the substrate texture provides a template for the epitaxial growth of the crystalline superconductor material. Further, the substrate provides structural integrity to the superconductor layer.
Substrate 110 and/or buffer 120 can be textured to provide a template that yields an epitaxial superconductor layer 130 with excellent superconducting properties such as high critical current density. Materials such as nickel, copper, silver, iron, silver alloys, nickel alloys, iron alloys, stainless steel alloys, and copper alloys can be used, among others, in the substrate. Substrate 110 can be textured using a deformation process, such as one involving rolling and recrystallization annealing the substrate. An example of such a process is the rolling-assisted biaxially textured substrate (RABiTS) process. In this case large quantities of metal can be processed economically by deformation processing and annealing and can achieve a high degree of texture.
One or more buffer layers 120 can be deposited or grown on the surface of substrate 110 with suitable crystallographic template on which to grow the superconductor layer 130. Buffer layers 120 also can provide the additional benefit of preventing diffusion over time of atoms from the substrate 110 into the crystalline lattice of the superconductor material 130 or of oxygen into the substrate material. This diffusion, or “poisoning,” can disrupt the crystalline alignment and thereby degrade the electrical properties of the superconductor material. Buffer layers 120 also can provide enhanced adhesion between the substrate 110 and the superconductor layer 130. Moreover, the buffer layer(s) 120 can have a coefficient of thermal expansion that is well matched to that of the superconductor material. For implementation of the technology in commercial applications, where the wire may be subjected to stress, this feature is desirable because it can help prevent delamination of the superconductor layer from the substrate.
Alternatively, a non-textured substrate 110 such as HASTELLOY, a corrosion resistant alloy manufactured by Haynes International, Inc. (Kokomo, In) can be used, and textured buffer layer 120 deposited by means such as the ion-beam-assisted deposition (IBAD) or inclined substrate deposition (ISD). Additional buffer layers 120 may be optionally deposited epitaxially on the IBAD or ISD layer to provide the final template for epitaxial deposition of an HTS layer 130.
By using a suitable combination of a substrate 110 and one or more buffer layers 120 as a template, superconductor layer 130 can be grown epitaxially with excellent crystal alignment or texture, also having good adhesion to the template surface, and with a sufficient barrier to poisoning by atoms from the substrate. The superconductor layer 130 can be deposited by any of a variety of methods, including the metal-organic deposition (MOD) process, metal-organic chemical vapor deposition (MOCVD), pulsed laser deposition (PLD), thermal or e-beam evaporation, or other appropriate methods. Lastly, a cap layer 140 can be added to the multilayer assembly, which helps prevent contamination of the superconductor layer from above. The cap layer 140 can be, e.g., silver or a silver-gold alloy, and can be deposited onto the superconductor layer 130 by, e.g., sputtering. In the case where slitting is performed after lamination, the cap layer may also include an additional laminated metal “stabilizer” layer, such as a copper or stainless steel layer, bonded to the cap layer, e.g., by soldering, forming a gap layer.
An exemplary as-fabricated multilayer HTS wire 100 includes a biaxially textured substrate 110 of nickel with 5% tungsten alloy; sequentially deposited epitaxial buffer layers 120 of Y2O3, yttria stabilized zirconia (“YSZ”), and CeO2; epitaxial layer 130 of YBCO; and a gap layer 140 of Ag. Exemplary thicknesses of these layers are: a substrate of about 25-75 micrometers, buffer layers of about 75 nm each, an YBCO layer of about 1 micrometer, and a gap layer of about 1-3 micrometers. HTS wires 100 as long as 100 m have been manufactured thus far using techniques such as those described above.
During use, it is desirable that the HTS wire is able to tolerate bend strains. A bend induces tensile strain on the convex outer surface of the bend, and compressive strain on the concave inner surface of the bend, thereby subjecting the HTS layer to tensile or compressive strains depending on the direction in which the wire is bent. While a modest amount of compressive stress can actually enhance the current carrying capacity of an HTS layer, in general subjecting the whole assembly to stress (especially repeated stress) places the wire at risk of mechanical damage. For example, cracks could be formed and propagate in the HTS layer, degrading its mechanical and electrical properties, or the different layers could delaminate from each other or from the substrate.
Methods for reducing stress in the HTS layer are described, e.g., in U.S. Pat. No. 6,745,059 and U.S. Pat. No. 6,828,507. For example, a copper strip, chosen to have similar thickness and mechanical features to the substrate, can be bonded onto the upper surface of the insert. This sandwiches the HTS layer roughly in the middle of the overall structure, so if the assembly is bent, the HTS layer is neither at the outer nor inner surface of the bend. Two of these assemblies can also be bonded together at their respective copper strips to form a single HTS wire assembly. In this case, the two substrates face outward, and the copper tapes are in the middle of the assembly. In this case the inclusion of a second assembly provides additional current carrying capacity; however, electrical contact to the HTS layers requires splicing the wire open, or partially removing one of the inserts in the contact section.
A further issue for coated conductor HTS wires is that of environmental contamination when the wire is in use. Environmental exposure can slowly degrade the electrical performance of HTS layers. Also, in the presence of cryogenic liquids such as liquid nitrogen in contact with the wire, the liquid can diffuse into pores within the wire, and on warming can form “balloons” that can damage the wire. Sealing the wire is desirable to prevent either environmental exposure of the HTS layers or penetration of the liquid cryogen into the wire. Seals for HTS assemblies are described in, e.g. U.S. Pat. No. 6,444,917.
The coated conductor approach has been greatly advanced in recent years to the point where long length manufacturing of reinforced tapes is being established. However, the utility of these tapes would be greatly increased if they could be made to any required length via low resistance joints that are mechanically robust and conform to tight geometric tolerances.
HTS wires must be joined in the field to each other and to terminations and leaders. As well, yield and wire quality can improve with factory splicing, thereby reducing wire price and enabling shipment of wires of lengths beyond inherent manufacturing limits. These splices must meet similar requirements as the wire.
Early splices were lap joints. A lap joint is a process of joining two pieces of material by overlapping them. Thus, in the case of HTS wires, two HTS wires can be joined by overlapping the ends of the wires over a set distance and then soldering the wires together. The lap joint method creates a splice that is about 2.0 to 2.2 times the thickness of the original wire.
Although the lap method is feasible for first generation wires or tapes, the second generation tape, where the insulating layer is between the YBCO film and substrate, requires the use of a face-to-face strap or a conductive bridge with a lap joint at each end to retain the original orientation of the parent wire being spliced and to minimize splice resistance. Co-pending U.S. application Ser. No. 11/880,586 discloses the conductive bridge splice in greater detail and the disclosure of which is hereby incorporated by reference in its entirety. However, the conductive bridge structure takes twice as long to make as the simple lap joint, because essentially two laps joints must be made, e.g. one for each end of the strap, doubles splice resistance, and introduces two lumps in the wire per splice. As with first generation wire, if the conductive bridge is the same material as the parent, then the splice will typically be 2.0 to 2.2 times the thickness of the parent wire. The splice should be similar in thickness and mechanical properties to the parent wire the better for cabling, as a stiffer thick region requires greater tension in cabling increases the likelihood of splice splitting in bending, and as short a length as possible to minimize local deviations from cable pitch. These issues are aggravated by the need for thicker lamination strips for adequate wire stabilization in the cable. Therefore, a need exists for a physically and mechanically near symmetric (two-sided), mechanically robust, yet compliant, splice that requires only one joint per splice, while still preserving the mechanical and electrical properties of each of the spliced wires.